Bought this Mixteca Culture coin recently from MA-SHOPs: It has the same large 1000 fineness stamped between the denomination characters as does the Paraguay 150 Guaranies KM# 36 1972 Proof. However, another example from NumisMaster shows a much smaller fineness between the denomination characters for the coin: Is there an actual advantage to creating a slab which documents the variant when the variant is so plainly visible and the normal coin so well documented?
I do not see a variation in my 2019 Krause World Coin Catalog. KM#65, .999 silver, mintage-estimated 10,000 price/value PF63-$45.00 PF65-$60.00
Yes, NumisMaster is the digital arm of Krause Publications, launched in 2008. See the difference between the two coins? In the reference, the "1000" printed above "GUARANIES" is so small it looks like "GUARĀNIES" with a macron over the letter A!
Found links to the Catálogo Virtual de Monedas from the MONEDAS DEL PARAGUAY 1790 - 2015 / PARAGUAYAN COINS web page at https://www.portalguarani.com/museos.php?pormustytr=MTc=. In this catalog, the 1972 - 150 GUARANÍES (PLATA)... reverse images all share the large fineness font: The 1973 - 150 GUARANÍES PLATA)... reverse images all share the small fineness font: The 1974 - 150 GUARANÍES (PLATA)... reverse images all share the large fineness font as in the 1972 series: For the 1975 - 150 GUARANÍES (PLATA)... series of coins, they all share a reverse image of the large fineness font above the denomination characters: Contacted nisa@pla.net.py for more information concerning any known varieties.
Took these photos of the coin: Contacted Miguel Pratt, author of COINS AND ESSAYS OF PARAGUAY (3rd EDITION) ISBN: 978-99953-2-544-2. The now defunct mint was located in Berlin. No photographs of the 1973 cultural coin obverse surfaces, having the date, denomination and purity, are presented in the book. The website does have an obverse image, however, the image is that of the same obverse repeated for the entire 1973 series of 150 Guaraínes coins. It appears that the coin die had the purity tolerance double stamped with a faint "1000" purity behind the "1O00," perhaps to optically center the purity above the letter 'A' prior to the decision to make the purity a half size smaller.
A similar variation can be found in the Paraguay 150 Guaranies KM# 37 1973 Proof. This is a photo of the large purity: This is a photo showing a half size purity typeface:
